1. Field of the Invention
The present invention relates to an exposure controller for an on-vehicle camera mounted on a vehicle, which is capable of controlling an exposure of the on-vehicle camera such that the on-vehicle camera is capable of photographing a subject with a constant contrast regardless of variation of peripheral brightness.
2. Description of the Related Art
Recently, a lane deviation alarm system has been proposed, which has a camera mounted on a vehicle, and which detects a white line or a yellow line (hereinafter, referred to as a lane marker) from an image obtained by imaging a lane marker drawn on edges of a driving lane by the mounted camera by using image processing, and judges a possibility of a lane deviation based on a position of the lane marker and behavior of the vehicle to output an alarm.
In the system using such an image processing, it is preferable that the lane marker can be steadily detected regardless of peripheral brightness.
Therefore, an exposure controller has been proposed, which performs exposure control of a camera according to peripheral brightness (see Japanese Patent Application Publication No. 2006-60504).
The technology disclosed in Japanese Patent Application Publication No. 2006-60504 includes calculating an average pixel value of a road surface area, multiplying the calculated average pixel value by a constant according to the average pixel value to correct the average pixel value, performing an exposure control such that the corrected average pixel value is equal to a target pixel value to image an image, and detecting a lane marker from the image imaged as described above.
However, in the exposure control method described in Japanese Patent Application Publication No. 2006-60504, it is not considered that an exposure property of an image pickup device used in an imaging part has non-linearity. Therefore, in the image on which the exposure control has been performed, contrast between the road surface and the lane marker is not constant.
Here, when detecting the lane marker from the image on which the exposure control has been performed, it is necessary to set an appropriate threshold value for distinguishing the lane marker from the road surface in each imaged image. However, in case where the contrast is not constant as described above, it is necessary to change the threshold value in every image so as to appropriately distinguish the lane marker and the road surface. Accordingly, there is a problem in that process for setting threshold values is complicated and calculation time is increased.
Furthermore, Japanese Patent Application Publication No. 2006-60504 discloses an exposure control method when peripheral brightness is drastically increased but not an exposure control method when peripheral brightness is drastically decreased, that is, it gets drastically dark. Therefore, it is not obvious whether or not the technology disclosed in Japanese Patent Application Publication No. 2006-60504 can apply to the case where it gets drastically dark, such as the vehicle enters a tunnel.